This invention relates to a circuit arrangement for igniting and supplying a lamp with a DC current, comprising
input terminals for connection to terminals of a power supply source supplying a DC voltage, PA1 a DC-DC converter coupled to the input terminals and provided with PA1 a first inductive element, PA1 a first unidirectional element, PA1 a first switching element, PA1 a control circuit coupled to a control electrode of the first switching element for rendering the first switching element conducting and non-conducting at a frequency f, PA1 output terminals for connecting the lamp, and PA1 a first circuit which connects the output terminals during operation and comprises a second switching element and means for rendering the second switching element conducting and non-conducting. PA1 a light-transmissive discharge vessel having a filling comprising a rare gas, and two electrodes, PA1 a lamp housing secured to the discharge vessel, PA1 a lamp cap having electric contacts and secured to the lamp housing, and PA1 a ballast circuit coupled between the contacts and the electrodes for generating a lamp current from a power supply voltage.
The invention also relates to a compact lamp.
A circuit arrangement as described in the opening paragraph is known from U.S. Pat. No. 5,581,161. In the known circuit arrangement, the DC-DC converter is constituted by a down-converter. The second switching element is conducting immediately after the known circuit arrangement is put into operation. The control circuit renders the first switching element conducting and non-conducting at a frequency f. During operation of the known circuit arrangement, both a first and a second lamp electrode form a part of the first circuit. The first inductive element and the first circuit convey current during a first time interval, so that the electrodes of the lamp connected to the circuit arrangement are preheated. At the end of the first time interval, the second switching element is rendered non-conducting, so that the first circuit no longer conveys current. The first inductive element subsequently generates an ignition voltage. After ignition of the lamp and during stationary operation, the second switching element remains non-conducting and the control circuit renders the first switching element high-frequency conducting and non-conducting, and a lamp connected to the circuit arrangement is fed with a DC current supplied by the down-converter. Since the down-converter consists of only a small number of components, the known circuit arrangement can be manufactured relatively easily and thus also at low cost. A drawback of the known circuit arrangement is, however, that the first inductive element must be dimensioned in such a way that the lamp ignites reliably and rapidly. Since the first inductive element also conveys current during stationary operation of the lamp, the power losses occurring in the first inductive element during stationary operation should be relatively low. For these reasons, the first inductive element is often relatively large in practice and is expensive. Moreover, the ignition process-dictated dimensioning of the first inductive element is often not optimal for stationary lamp operation.